The subject of the present invention is a process for preparing polyisocyanates-polyisocyanurates by catalytic cyclotrimerization of polyisocyanate monomers. It relates more particularly to the cyclotrimerization of isocyanate functional groups carried by sp3-hybridized carbons of a particular kind. These carbons are carbons either in the neopentyl position, that is to say said carbon is linked to a tertiary radical such as tert-butyl, or said carbon is a secondary or tertiary carbon, preferably belonging to an aliphatic ring.
The cyclotrimerization of isocyanates has been known for several decades. Such trimerizations generally involve mechanisms which are basic by nature. These bases may be ionic by nature, such as hydroxides or salts of a strong base and strong acid. These bases may also be nonionic bases but may have a very accessible and highly basic electron pair.
The first trimerizations carried out were on aromatic isocyanates, that is to say on isocyanates carried by one of the member carbons of an aromatic ring. Such trimerizations do not pose any particular difficulty and suitable catalysts are readily found for easily carrying out trimerizations of aromatic isocyanates.
The problems inherent with aromatic isocyanates have led to the development of aliphatic isocyanates which have chemical and physical properties which are different and often much improved over aromatic isocyanates.
Most aliphatic isocyanates are too volatile to be used as such. It is therefore necessary to increase their molecular weights by producing either oligomers (biurets, trimers, etc.) or oligo-condensates with polyols.
The trimerization of linear-chain aliphatic isocyanates is now well controlled and there are available catalysts capable of carrying out trimerizations efficiently, that is to say with good yields and developing little coloration.
On the other hand, for reasons which are not fully elucidated, the trimerization of branched aliphatic isocyanates, and especially of cycloaliphatic isocyanates or those in the neopentyl position, remains difficult because, on the one hand, of the difficulty of finding catalysts giving satisfactory yields and satisfactory productivity and because, on the other hand, these derivatives have a strong propensity to develop within them undesirable coloration.
Coloration problems, as mentioned above, have already been treated in the case of linear aliphatic isocyanates such as HDI (hexamethylene diisocyanate), but the techniques used are not easily transposable to branched-type isocyanates, such as isocyanates in neopentyl, cycloaliphatic and tertiary positions, in particular because the reactivity of these branched isocyanates is very significantly less than that of linear isocyanates, with the result that the catalysts used for linear aliphatic isocyanates often give low, or even very low, yields in the case of branched isocyanates.
Thus, it has been proposed to use as basic anion a fluoride anion for carrying out cyclotrimerizations of linear aliphatics without coloration, but this technique cannot easily be used for cycloaliphatics since the yields are very significantly lower in their case.
The degassing of isocyanate monomers has also already been described for the purpose of cyclotrimerizing HDI.
Thus, EP 330 966 discloses a process for trimerizing HDI with the help of a quaternary ammonium hydroxide in which the starting HDI is stripped of carbon dioxide down to a residual content of less than 20 ppm by weight, so as to reduce the amount of catalyst to less than 0.03% by weight for the purpose of reducing the coloration of the final reaction mixture.
EP 524 501 discloses a process for preparing polyisocyanates comprising isocyanurate groups and allophanate groups from HDI by using, as catalyst, a trimethylbenzylammonium hydroxide or a quaternary ammonium hydroxide in which the substituents are C1-C20 alkyl groups optionally substituted with hydroxyl groups. It is specified in that document that the starting HDI mixture contains less than 10 ppm CO2.
However, the catalysts illustrated in that document have in three cases methyl substituents, the fourth substituent being either a benzyl group, or a hydroxyalkyl group.
U.S. Pat. No. 5,232,988 discloses a process for preparing blocked polyisocyanates comprising a trimerization reaction on cyclic diisocyanates such as IPDI in the presence of quaternary ammonium carboxylates, phenolates or hydroxides in which the starting diisocyanate is treated by bubbling into it an inert gas.
U.S. Pat. No. 5,914,383 discloses the preparation of a polyisocyanate composition comprising trimers of the iminooxadiazine-dione type and optionally isocyanurate groups in the presence of a polyfluoride as catalyst.
That document discloses in particular the preparation of a composition of the aforementioned type from HDI and IPDI, in which, before the proper catalytic reaction, the gases dissolved in the starting isocyanate mixture are removed.
DE 19 754 748 and EP 927 731 disclose processes for preparing polyisocyanates from IPDI using starting monomers having low chlorine contents, obtained especially by what is referred to as the xe2x80x9cureaxe2x80x9d process.
On the other hand, EP 379 914 and U.S. Pat. No. 5,013,838 recommend the addition of carbon dioxide during trimerization of aliphatic and/or cycloaliphatic organic diisocyanates, using a catalyst consisting of an ammonium or phosphonium fluoride.
This is why one of the objectives of the present invention is to provide a process which allows branched isocyanate trimers to be obtained while preventing the development of undesirable coloration.
Another object of the present invention is to provide a process which allows good yields and good productivity to be achieved.
Another objective of the present invention is to provide a process which allows the trimerization of cycloaliphatic isocyanates and especially of IPDI (often referred to as isophorone diisocyanate).
Another objective of the present invention is to provide a process which allows the trimerization of cycloaliphatic isocyanates, by improving the reactivity of the catalytic system used for this purpose.
These objectives, and others which will appear later, are achieved by means of a process for trimerizing branched isocyanate functional groups by the action of a quaternary-ammonium- or quaternary-phosphonium-based catalyst, of which the sum of the carbons is at most equal to 30 and at least equal to 12; of which the counterion is chosen from anions corresponding to weak acids whose pKA is at least equal to 8, preferably 10 and more preferably 12; of which the substituents are aliphatic (that is to say they are linked to the atom carrying the positive charge via an sp3-hybridized carbon atom) and do not possess unsaturation in the beta position (as the benzyl, allyl or propargyl positions have); and in that said oniums comprise at most 2 radicals of more than 6, advantageously more than 10 and preferably more than 12 carbon atoms.
The subject of the invention is also a process for preparing polyisocyanates by cyclotrimerizing isocyanate functional groups carried by isocyanates having a branched hydrocarbon backbone, characterized in that it comprises the following steps:
a) supplying of starting isocyanate monomers;
b) removal of the reactive gases from said isocyanate monomers;
c) optionally, supplying of a starting reaction mixture comprising said starting isocyanate monomers;
d) addition of a cyclotrimerization catalyst consisting of a quaternary ammonium or phosphonium compound, as defined above;
e) reaction until the desired degree of conversion obtained; and
f) optionally, removal of the monomers that have not reacted;
the order of steps b) and c) not mattering.
Thus, removal of the reactive gases may take place on the starting isocyanates themselves or after these have been introduced into a possible reaction mixture comprising a suitable solvent.
It goes without saying that when the reaction is carried out with no solvent, directly in the mass of isocyanates, step c) may be omitted.
Advantageously, no substituent of the xe2x80x9coniumxe2x80x9d (ammonium or phosphonium) compounds carries a hydroxyl functional group (such as a phenol or alcohol).
Furthermore, according to one of the preferred embodiments of the present invention no substituent has fewer than 2, advantageously fewer than 3, carbon atoms.
According to another preferred method of implementation, all of the substituents have at least 1 and at most 2 radicals of more than 12, advantageously more than 10 and preferably more than 6 carbon atoms.
The substituents are advantageously chosen from alkyls, including aralkyls and cycloalkyls. It is preferable that they be not branched in the alpha position or even in the beta position. It is also desirable that they be not functionalized. However, ether functional groups are acceptable, and especially those of the type arising from the opening of the epoxides by an alcohol and an alcoholate.
According to the present invention, are considering as acids not only the usual protic acids but also water and alcohols and any compound that can undergo the extraction of a proton.
The preferred acids are essentially water and alcohols, which means that the counterions of the quaternary ammonium are introduced in the form of hydroxide or alcoholate.
However, it would not be outside the scope of the present invention to manufacture in situ compounds of this type. This is because, during the research which led to the present invention, it was discovered that the structure of the ammonium or phosphonium plays an important role in the development of colorization subsequent to the polymerization reaction.
It turns out in particular that the presence of methyl and of benzylic by nature compounds plays a deleterious role as regards the coloration of the composition after trimerization.
The best results have been obtained when the identical or different radicals substituting the ammonium or phosphonium are chosen from propyl, butyl and pentyl, these radicals preferably being all linear.
The present invention is aimed at the trimerization of polyisocyanate compounds in general and, preferably, those carrying two isocyanate functional groups, said polyisocyanate compounds being denoted in the present description by isocyanate monomers.
However, during the reaction other compounds may be formed depending on the nature and the formulation of the catalyst and, where appropriate, on the cocatalyst, as disclosed in Application WO 99/23128, especially dimers, aminooxidiazine diones, carbamates, allophanates, biurets, etc.
Although the present invention may be aimed at the trimerization of branched monomers with unbranched, that is to say linear, monomers, such as HDI, it is mainly aimed at the trimerization of or the trimerization between different compounds belonging to the same family of branched monomers.
The term xe2x80x9cbranched monomersxe2x80x9d, as already mentioned, is understood to mean either a monomer of which at least one isocyanate functional group is in a cycloaliphatic, secondary, tertiary or neopentyl position. These monomers are advantageously cycloaliphatic monomers.
Thus, among these products, those which are preferred are cycloaliphatic. These monomers are advantageously such that at least one, advantageously both, isocyanate functional group(s) is, or are, distant from the closest ring by at most one carbon and preferably linked directly to it. Furthermore, these cycloaliphatic monomers advantageously have at least one, preferably two, isocyanate functional groups chosen from secondary, tertiary or neopentyl isocyanate functional groups.
The best results are obtained when the cycloalipiatic monomer has a low degree of conformational freedom. As monomers capable of giving good results, mention may be made, for example, of the following monomers:
compounds corresponding to the hydrogenation of the aromatic ring or rings carrying the isocyanate functional groups of aromatic isocyanate monomers and especiall y TDI (toluene diisocyanate) and diisocyanatobiphenyls, the compound known by the abbreviation H12MDI, the various BICs [bis(isocyanatomethylcyclohexane)] and cyclohexylene diisocyanates, these being optionally substituted;
and most particularly
norbornane dtsocyanate, often denoted by its abbreviation NBDI;
isophorone diisocyanate, or IPDI, or more specifically 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexyl isocyanate.
The starting monomers may also be products from the oligomerization of low-molecular-mass isocyanates, these oligomerization products carrying isocyanate functional groups. In this case, it is unnecessary to separate the unconverted oligomer from the reaction product formed.
The work by the inventors, who are the authors of the present invention, has allowed them to discover that the polymerization of isocyanates of the type described above may be carried out with a high yield, and a substantially reduced coloration, by the combination of a catalyst of the type described below and a step in which the reactive gases dissolved in the reaction mixture, comprising the starting monomers on which the subsequent polymerization reaction is carried out, are removed.
The term xe2x80x9creactive gasesxe2x80x9d is understood to mean the gases capable of interfering with the catalytic reaction of polymerization, especially trimerization, as opposed to inert gases.
These are especially CO2, but also reactive gases from the air, that is to say the gases other than nitrogen and rare gases, especially oxygen, together with chlorine and possibly chlorinated volatile compounds coming especially from the phosgenation step.
The inventors have also determined that although the sensitivity of the reaction is influenced by the content of CO2 dissolved in the reaction mixture, other gases present, especially air, do have a substantial influence on the way the reaction evolves. Thus, the removal of the reactive gases greatly favors the trimerization reaction, even when the CO2 content of the starting monomers is as low as of the order of 1 to 2 ppm.
The inventors have also found that the content of chlorinated compounds, defined by the content of total chlorine, has only a minimal influence on the sensitivity of the reaction.
The reactive gases may be removed by any means known to those skilled in the art, especially by creating a vacuum, by diffusing in an inert gas or else by the use of molecular sieves or chemical traps, or a combination of one or more of these methods.
When the starting isocyanate monomers have been stored for a long time, generally more than two months, which results in an increase in the dissolved reactive gases, it is generally preferable to remove the dissolved reactive gases by subjecting the reaction mixture to a vacuum followed by the introduction of an inert gas.
The vacuum is generally less than 1xc3x97104 Pa (100 mbar).
When the starting monomers have been stored for a shorter time, the introduction by diffusion into the liquid reaction mixture of an inert gas is generally sufficient.
Advantageously, the inert gas is nitrogen or a rare gas, for example argon. The diffusion is carried out at a temperature of around 0 to 70xc2x0 C. Higher temperatures may be used.
The diffusion may be carried out by sparging. However, the degassing is all the more effective as the size of the gas bubbles generated is small. For this purpose, the diffusion tools will be adapted so that the removal of the reactive gases is rapid, without causing the diffusion devices used to he clogged.
The creation of a vacuum and/or the diffusion are carried out for a time long enough to lower the amount of reactive gases in the starting reaction mixture. In particular, degassing is considered to be proper when the dissolved CO2 is less than 20 ppm, and preferably less than 2 ppm.
The amount of CO2 and the content of gases other than CO2 are advantageously determined by conventional analytical techniques, especially by gas chromatography, using for standards profiles obtained with injections of variable volumes of pure CO2 and/or other pure gases.
The optimum degassing conditions also depend on the conditions under which the isocyanates are treated after they have been synthesized, in general by phosgenation, and on the storage conditions.
In particular, the degassing operation may be carried out after phosgenation of the amines, once the mixture has been purified and has stabilized, under the conditions below. If it is kept under proper conditions (in the dark, at a temperature below 30xc2x0 C. in airtight and especially moisture-tight containers), the isocyanate monomer may be used in the trimerization process even after a prolonged stored period (two to four months, or longer).
However, to be sure of obtaining low coloration, it is preferable to treat the isocyanates stored for a long period, as described below.
Advantageous conditions for removing the quantities of dissolved reactive gases are, in the case of quantities of starting monomers of around 500 g, after they have been stored for a period of greater than two months, a vacuum of 15 mbar for a long enough time, generally one hour at room temperature, followed by diffusion of an inert gas, for example nitrogen, by sparging or the like into the liquid or the mass for about 30 minutes.
For starting monomers stored for a shorter period of time, simple diffusion of nitrogen, generally by sparging, for a time of 30 minutes at room temperature is sufficient, for the same quantity of starting monomers.
If the gas to be removed is highly acidic, that is to say has a pH value of less than 3 (Handbook, 67th Ed., p D-146 of Chemistry and Physics), it is preferable to carry out the degassing before the introduction of the catalyst, in order to prevent irreversible poisoning of the catalyst.
Conversely, when the gas to be removed is not highly acidic, in particular in the case of CO2, the degassing may be carried before or after the catalyst has been introduced and also during the step of trimerizing the starting isocyanates.
The inventors have also shown that the formulation of the catalyst also has an influence on the reaction kinetics. In particular, in the case of aqueous formulations, water reacts with the isocyanate to give biuret and CO2. The CO2 generated would result in a reduction in the reaction rate. This is why it is preferable to maintain continuous diffusion of inert gas when aqueous catalyst formulations are used.
This also makes it possible to use very small quantities of catalyst and avoids the successive introduction of small quantities of catalyst in order to maintain the reactivity.
Aqueous or hydroalcoholic formulations are preferred, on account of the solubility of oniums in these media before introduction into the reaction mixture, due to the hydrogen dissolved in these solutions.
The operating conditions for step e) of the process according to the present invention may be those which are usually employed for the trimerization of linear aliphatics, in particular the temperature may be chosen between 40 and 100xc2x0 C., preferably between 50 and 90xc2x0 C.
A particularly satisfactory operating point lies between 60 and 80xc2x0 C.
It is also possible to start to heat the reaction mixture before the end of step b) in which the dissolved reactive gases are removed, particularly when said step consists of simply sparging an inert gas.
During the study which led to the present invention, it was shown that other parameters have an influence on the reduction in coloration. The effects of these parameters are favorable by themselves or in combination with the other parameters in the present application, which were mentioned above.
Thus, it has been demonstrated that prior distillation of the starting monomers makes it possible to reduce the coloration developed during the trimerization reaction.
Usually, the trimerization reactions using quaternary oniums are blocked by heating. During the study which led to the present invention, it was shown that this catalyst deactivation technique gives very poor results as regards coloration. It was therefore shown to be preferable to use another technique, known for linear alciphaics, namely the addition of a high-strength acid such as sulfonic acids, for example mesylic, tosylic or phosphoric acids, or a medium-strength acid, preferably an ester of an acid containing phosphorus. As esters of acids containing phosphorus, mention may be made of phosphate esters, especially monoesters and diesters. Mention may also be made of phosphonates, preferably monoesterified ones, and phosphinates.
Poisoning of the catalyst is, however, preferably achieved by means of acid esters of phosphoric acid, and especially dialkyl phosphoric esters, in particular such as dibutyl phosphate and di-2-ethylhexyl phosphate.
The manner of introducing the catalyst according to the invention also plays a role. Thus, when the catalyst is introduced into an aqueous mixture, the coloration develops less than in the case in which the catalyst is introduced into an alcoholic mixture.
However, hydroalcoholic solvents give quite good results.
To prevent regions of reaction runaway, it is desirable to introduce the catalyst in the form of a solution with a proper dilution by mass. Good results are given with 0.5 wt % to 40 wt % catalyst solutions.
The diluants must, on the one hand, ensure the dissolution of the ionic compound formed by the catalyst and, on the other hand, ensure diffusion into the medium formed by the monomers of the reaction mixture. Mixtures of water (0 to 50%, advantageously 5 to 40%), butanol (0 to 50%, preferably 5 to 40%) and heavy (greater than or equal to C8) alcohol(s) or alcohol ethers (2-ethoxy-ethylene glycol) (qsp 100%) give good results.
Although it is preferable to work using relatively dilute catalyst solutions, it is possible, according to the present invention, to use highly concentrated catalyst solutions provided that they are introduced at low temperature, advantageously at room temperature or preferably below room temperature, and provided that homogenization takes place before the process of heating the reaction mixture is initiated.
In general, the trimerization is carried out to the desired degree of conversion. For obvious economic reasons, it is preferred to stop the reaction at degrees of conversion of the isocyanate functional groups of at least 5% and preferably between 10 and 50%. At high degrees, greater than 50%, the polyisocyanates generally have a high viscosity not easily compatible with the subsequent application conditions. Once the polymerization reaction has stopped, the rest of the monomers are generally removed, by evaporation or distillation on a thin film and under a high vacuum.
The amount of catalyst used is advantageously between 10xe2x88x925 and 10xe2x88x922 times the total mass of the isocyanate monomers introduced, preferably 10xe2x88x924 to 10xe2x88x923, expressed as amounts by mass of the catalyst without solvent In the case of oligomers, amounts of catalyst such that the ratio of the mass of catalyst to the mass of isocyanate functional groups is between 10xe2x88x925 and 10xe2x88x922 are preferred.
The present invention is also aimed at compositions comprising:
20 wt % to 80 wt % of compound(s) having isocyanurate rings produced from cycloaliphatic monomer(s);
10xe2x88x926 to 10xe2x88x922 of a quaternary ammonium or phosphonium, the sum of the carbons of which is at most equal to 25 and at least equal to 15 and no substituent of which has fewer than 2, advantageously 3, carbon atoms;
20% to 80% of cycloaliphatic monomer (s);
and having a Hazen color value, relative to the percentage by mass of compounds) containing isocyanurate rings, of at most 1 Hazen, advantageously at most 0.75 Hazen and preferably at most 0.50 Hazen.